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International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056

Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072

ENHANCING NEAR-SURFACE SOIL SHEAR STRENGTH BY USING PINE

NEEDLES

Saurav1 , Dr. Vijay Kumar Bansal2

1 Research Scholar, Dept. of Civil Engineering, National Institute of Technology, Hamirpur, India

2 Associate professor, Dr. Vijay Kumar Bansal, Dept. of Civil Engineering, National Institute of Technology, Hamirpur, India

Abstract - This paper explores howpine needles canbeused as a natural and environmentally friendlyresource to increase shear strength of near-surface soils in hilly areas like Himachal Pradesh. Pine needle, which is in plenty and, in most cases, contributes to the environmentalproblemsuch as forest fire and pollution, is investigated as a sustainable geotextile solution. The study aims to determine the physical and mechanical characteristics of soil and pine needles using different laboratory tests, such as grain size analysis, specific gravity, liquid limit, standard Proctor test, and direct shear test. Four different lengths of pine needles (1 cm, 1.5 cm, 2 cm and full length) were added to the plain soil and compared. The findings reveal that the presence of soil alone is more shear strength, but when pine needles are added, ductilityand brittleness are enhanced. It was noted that the longer the pine needles the better the shear strength and the best the fibres used in the tests were the 2 cm ones. The paper ends with the conclusion that pine needles would be an effective reinforcing fibre when added to soil and can be used to offer the sustainable approach to soil stabilization, as well as to overcome the environmental issues associated with pine litter deposits.

Key Words: Pine needles , OMC , MDD , Direct shear test, Standard Proctor.

1. INTRODUCTION

In hilly regions, where soil erosion, landslides and slope instabilityisprevalent,soilstabilityiscrucial.Inplacessuch as Himachal Pradesh, we need stable soil for safe construction, agriculture and ecosystem preservation[1]. Typically, engineers strengthen soil by using synthetic materials, but these materials are expensive and not environmentallyfriendly.So,thereisaneedfornaturaland bio-friendlyalternatives.Thesedaysnaturalfibreslikecoir, jute,andhempareusedtoimprovesoilstrengthastheyare inexpensive,widelyavailableandeco-friendly.Pineneedles are also a natural resource and are readily available and foundinabundanceinhillyregionsduetothepresenceof chirr (Pinus roxburghii) pine trees. But they are not commonlyusedandcauseissuessuchasforestfiresandlow rate of decomposition. Pine needles are fibrous and their

surfaceis notsmooth,so theycan help in cohesion of soil particles and improving its strength. Pine needles used in soilcannotonlyincreasethestabilityofsoilbutalsohelpin preventingenvironmentalissuesarisingfromtheirdisposal. The study will evaluate methods to improve the shear strengthofshallowsoilusingpineneedles.Varioustestsare performed to assess the characteristics of soil and pine needlesandtesttheircombinedbehavior[2].Theobjectiveis to develop a cost-effective, easy and environmentally sustainabletechniqueforsoilstabilizationinhills.

1.1 SCOPE OF THE PROJECT

The research will cover the scope of the study as follows: Thecurrentresearchaimsatassessingthepossibilityofpine needles as a reinforcing substance in enhancing the shear strengthofnear-surfacesoil.Theresearchisnarroweddown to lab-based research through the standard geotechnical tests.Basictestslikegrainsizeanalysis,specificgravityand moisturecontentareusedtocharacterizesoilsamplesused in this research and aid in understanding their physical properties before reinforcement. The influence of pine needlesonsoilbehaviorhasbeeninvestigatedbycarrying outdirectsheartestsundervaryingnormalstresseswhich enabled comparison of reinforced and unreinforced soil conditions.Thelengthofpineneedleshasbeenvariedtosee the variation in interlocking behavior and shear strength withspecialconsiderationtoshortandrelativelylongfibres. The research is limited to controlled laboratory settings; hence,theaspectsoffieldlevelperformanceandlong-term stabilityarenotdiscussedintheresearch.Thisstudyapplies mainlythegeological aspect tothehillystateasHimachal Pradeshwherethepineneedlesareinabundanceandthe geotechnicalissuesofsoilstabilityexist. Thetreatmentor alterationofpineneedlesbychemicalsorothermethodsis not part of this study and only untreated fibres are examined.

1.2 EXPERIMENTAL STUDY

The experimental study was conducted to investigate the effect of pine needle reinforcement on the shear strength behavior of near-surface sandy silt soil. The study mainly

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056

Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072

focused on comparing the engineering performance of unreinforcedsoilwithsoilreinforcedusingpineneedlesof differentlengths.Laboratoryinvestigationswereperformed according to ASTM and IS standard procedures to ensure accuracyandreliabilityoftheresults.Thematerialsusedin the study included sandy silt soil collected near National Institute of Technology Hamirpur, pine needles obtained fromChirrPine(Pinus roxburghii)trees,waterformoisture conditioning, and standard geotechnical laboratory equipment.Thesoilsamplewasovendried,pulverized,and passedthrougha4.75mmsievebeforetesting.Pineneedles werecleaned,dried,andcutintodifferentlengthsof1cm, 1.5cm,2cm,andfulllengthtostudytheeffectoffibrelength onsoilbehavior

1.3 Experimental Investigation

Asetoflaboratorytestssuchasgrainsizeanalysis,specific gravity test, liquid limit test, Standard Proctor Test, and directsheartestwerecarriedoutfollowingASTMstandards. The behaviour of unreinforced soil and its reinforced counterpart has been compared under different normal stresses by applying the direct shear test. The results showedthatpineneedlescausedasignificantincreaseinthe shearstrength,ductilityand interlockingbehaviourofthe soil.Itwasfoundthattheshearstrengthwashigherasthe lengthofthefibreincreasedandhence,thepineneedlesof2 cm length and the full length resulted in the higher shear strength. The study results indicate that pine needles are suitable as a low cost, biodegradable and sustainable reinforcementmaterialforsoilstabilization.

2. METHODOLOGY

2.1 Soil Sample

ThesoilsamplewascollectednearNITHamirpur,Himachal Pradesh.Soilwasovendried,groundandpassedthrougha 4.75mmsievepriortotesting

Table -1: Physical properties of soil

2.2 Pine needles (Reinforcement material)

Pineneedleswerecleaned,driedandcuttoadesiredlength of 1 cm, 1.5 cm, 2 cm and full length (~ 4 cm) for experimentation.

2 Pine needles.

Table-2 Mechanical properties of pine needles

PROPERTIES PINE NEEDLES

Length(mm) 15mm

Fig: 1 collected soil sample.

Fig:

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056

Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072

2.3 Laboratory test conducted

Test/ Procedure Standard / Method Purpose / Use in Study

Sieve AnalysisSet (ASTM D1140, D7928) Todetermine particlesize distribution.

Density Bottle IS:2720 (Part 3/Sec1) Todetermine specificgravity.

Standard Proctor Compaction (ASTMD(698) Todetermine OMCandMDD.

Casagrande Apparatus (ASTMD4318) Todetermine liquidlimit.

Oven and Weighing Balance (ASTMD2216). Formoisture content determination.

Direct Shear Apparatus (ASTMD3080) Tomeasureshear strength parameters(Cand ∠ϕ)

(1.) Standard proctortest fordetermination of OMC and MDD

Standard proctor test is a laboratory compaction test to measureOptimummoisturecontent(OMC),Maximumdry density(MDD)ofsoil.

Compaction curve data: Fromthecompactiondata,the inter-relationship between the moisture content and dry

densityofsoilscanbeascertained.Withincreaseinmoisture contentof8to12thedrydensityalsorises,by1.53to1.60 g/cc.Thisisbecausewithalittlewater,thesoilparticlesare closer together and thus denser. The highest value of dry densityis1.60g/ccandthisiscalledasOptimumMoisture Content(OMC).Beyondthis,thatisatmoisturecontentof14 percentand 16percent,the drydensitydecreases. Thisis because excess water is present in the space between particles that reduces compaction. So, the best moisture contentforsoiltobecompactedis12%.

Table- 3 Data of OMC and MDD

2.4 Direct Shear Test (Main experimental)

Themainexperimentaltestusedinthisstudywasthedirect sheartest,whichwasusedtoexaminethecharacteristicsof shearstrengthofreinforcedandunreinforcedsoil.

Thetestwasperformedunderdifferentnormalstresses:

Pine needles of varying lengths were mixed with soil samples,andshearstrengthparameterswereevaluated

Fig: 3 OMC and MDD of soil sample

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Volume: 13 Issue: 05 | May 2026 www.irjet.net

Fig: 4 Direct shear apparatus

2.5 Shear Strength Results:

(1) Unreinforced Soil

(4) Soil Reinforced with 2 cm Pine Needles

(2) Soil Reinforced with 1 cm Pine Needles

(5) Soil Reinforced with Full Length Pine Needles

(3) Soil Reinforced with 1.5 cm Pine Needles

2.5 Comparison of Shear Strength Values:

Table4showsthedifferencesinshearstrengthfordifferent normalstressesanddisplacement(1cm,1.5cm,2cm).Inall thecases,thevalueofshearstrengthalsoincreaseswiththe variationofnormalstressfrom0.5to1.5.Thismeansthat thesoilbecomesstrongestastheloadisincreased.Asforthe shear strength, it is also greater with every normal stress appliedatboth1cmand1.5cmand2cm.Asanillustration, thevalueat0.5normalstressincreasesby0.38to0.58and at 1.5 normal stress, it increases by 1.19 to 1.42. This illustrates that the higher the amount of soil movement beforefailure,thehighertheresistanceofthesoil.Fromthe overallinformationinthetable,itcanbeseenthatwhenthe loadisincreasedandthesoilisdisplaced,theshearstrength ofthesoilincreases;thisindicatesthesteadybehaviourof thesoil.

Table- 4

Stress

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056

Itisseenthattheshearstrengthisenhancedduetoaddition ofpineneedles.Theshearstrengthisgreaterforalllevelsof normalstressforreinforcedsamples.Theenhancementis higherforlongerfibres

3. COMPARISON AND RESULTS OF SHEAR STRENGTH:

3.1 Comparison of Material Behaviour:

Material Strength Level Behavior

Soil(Sand) High Dense,Frictional

Pine Needle 1 cm VeryLow Poorinterlocking

Pine needle 1.5 cm Low-Medium Moderate interlocking

Pine needle 2 cm Medium Betterfiber interaction

FullLength Veryhigh Max.reinforcement

3.2 Graphical Comparison

Testdata isusedtogetgraphical comparisontoshow the relationbetweennormalstressandshearstressinavisual way.ThenormalstressisplottedintheX-axisandthepeak shearstressontheY-axisinthisstudy.Thepointsplotted followastraightlinetrendthatisincreasing,suggestingthat the shear strength also increases with an increase in the normal stress. This is a straight line that signifies the behaviourofshearstrengthofthesoilandwhichisbasedon Mohr-CoulombFailureCriterion.

Fig:5 Shear of Unreinforced soil and reinforced soil with 1, 1.5 and 2cm pine needles.

Fig: 6 Shear strength of all pine needles.

1.Increaseinlengthofpineneedleleadstoincreaseinshear strength.

2.Theincreaseisnotlinear(atfirstitincreasesslowly,then fast).

3.Optimumstrengthisatfull-length(~4cm).

Thisconfirms:

 Betterinterlocking.

 Higherfrictionalresistance.

 Enhancedsoilreinforcingaction.

Shear stress versus normal stress plot for reinforced and unreinforcedsamplesshowsa linear relationship. But the slopeisgreaterforthereinforcedsamples,showingthatthe shearstrengthparametershavebeenimproved.

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056

Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072

4. CONCLUSION

The present research was undertaken to investigate the performanceofpineneedlesasareinforcementmaterialto enhance the shear strength of shallow soil. From the experimental studies, such as direct shear testing, index property testing and comparative analysis, the following conclusionsaremade:

1. Soil Characteristics:

Themainexperimentaltestusedinthisstudywasthedirect sheartest,whichwasusedtoexaminethecharacteristicsof shearstrengthofreinforcedandunreinforcedsoil.

2. Influence of Pine Needles:

Shearstrengthofsoilwasaffected bytheadditionofpine needles.Followingobservationsweremade:

•Shearstrengthisobservedtoincreaseasthenormalstress increases.

•Itwasfoundthatshearstrengthofthereinforcedsamples washigherthanthatoftheunreinforcedsoilsample.

3. Effect of Fibre length: Pineneedlelengthisanimportant factorinthereinforcementofsoil:

•1cm:Limitedimprovementduetopoorinterlocking.

•1.5cmlength:Averageimprovementwithimproved interaction.

• 2 cm length: Better improvement due to better interlockingandfriction.

• Full-length (~4 cm): Optimum shear strength and mixedproperly.

4. Shear Strength Improvement: Theresearchsuggestsa gradualimprovementinshearstrengthwithincreasingfibre length.Theimprovementinshearstrengthwithfull-length pine needles wasin the rangeof65-80% whencompared with the soil alone, which shows the effectiveness of pine needlesasreinforcingmaterial.

5. Soil vs Pine Needles:

•Soilalonehasgreaterstrengthduetohighpacking.

• Pine needles alone are weak due to high compressibility.

•Butsoil+pineneedlesexhibitshigheststrength.

6. Environmental Significance: The use of pine needles helpsin:

•Reducingforestwaste.

•Minimizingfirehazards.

•Encouraginggreenbuilding.

5. REFERENCES

[1]C. Chauhan and K. V. Uday, “Multiparameter study of shearstrengthimprovementnear-surfacebyvegetation rootsandfibers,” Arab. J. Geosci.,vol.18,no.1,2025.

[2]A.K.AnshuandR.P.Boruah,“Enhancedthesoilstrength usingPinetreeneedles,”IJEDR,vol.2,no.10,pp.119–122,2022.

[3]N.A.WagiminandN.K.A.Rahman,“Reinforcedsoft soilwithpineappleleaffiber,” peat,vol.3,no.2,pp. 172–182,2022.

[4]M. Stasiak et al., “Mechanical characteristics of pine biomassofdifferentsizesandshapes,” Eur. J. Wood Prod., vol.77,no.4,pp.593–608,2019. -Apr-2026].

[5]X.Wen-Jie,X.Qiang,andH.Rui-Lin,“Studyontheshear strengthofsoil–rockmixturebylargescaledirectshear test,” Int. J. Rock Mech. Min. Sci. (1997),vol.48,no.8,pp. 1235–1247,2011.

[6]G.L.SivakumarBabu,A.K.Vasudevan,andM.K.Sayida, “Use of coir fibers for improving the engineering propertiesofexpansivesoils,”J.Nat.Fibers,vol.5,no.1, pp.61–75,2008.